1. Field of the Invention
The present invention relates to a heat-shielding structure for use in electrical connector assemblies, e.g. for connecting a spark plug to an ignition cable in vehicle internal-combustion engines, such as used in automobiles. The invention more particularly concerns structures that ensure heat-shielding for such connector assemblies, while maintaining reliable connections between the spark plug and the ignition cable.
2. Description of Background Information
Heat-shield structures for such connector assemblies are already known, for example, U.S. Pat. Nos. 4,671,586 and 5,163,838.
FIGS. 1 and 2 show a known connector assembly, in which a plug boot 510 houses a high-voltage terminal (not shown in the figure) at the end portion of an ignition cable K. The plug boot 510 is then fitted to a stud terminal Pt of spark plug P, which projects from a cylinder block head H of the internal-combustion engine. In this manner, the ignition cable K is normally connected to the spark plug P.
The plug boot 510 may be covered with a heat-shielding sheath 520 made of metal. The heat-shielding sheath 520 shields the radiant heat produced by heat-generating sources such as an exhaust manifold, so as to protect the plug boot 510 from heat deterioration.
An end portion of heat-shielding sheath 520 is provided with a downwardly-projecting spring 530 (shown in FIG. 1). When the plug boot 510 is fixed to the stud terminal Pt of the spark plug P, the spring 530 is placed into contact with a grounded portion of the spark plug P, so that heat-shielding sheath 520 is also grounded. Accordingly, electrical discharges normally generated from heat-shielding sheath 520 can be prevented.
However, the heat-shielding assembly shown in FIGS. 1 and 2 usually does not provide a sufficient heat-shielding effect. For instance, when a heat source is located adjacent to cylinder block head H, or the heat source becomes very hot, the plug boot 510 cannot be efficiently protected from heat deterioration.
According to another known type of heat-shielding assembly, shown in FIGS. 3 and 4, the heat-shielding sheath 520 is further covered with an auxiliary heat-shielding sheath 540.
The auxiliary heat-shielding sheath 540 has a generally cylindrical shape which is closed at one end to form a base. The base includes a hole 540a, through which a spark plug P and a threaded portion Pa pass. After threaded portion Pa passes through the hole 540a, the spark plug P is screw-mounted into a cylinder block head H. In this state, the peripheral zone of the hole 540a is flanked by the spark plug P and the cylinder block head H, and fixed between these two parts. Thereafter, auxiliary heat-shielding sheath 540 covers the outer surface of heat-shielding sheath 520 at a predetermined distance from the auxiliary heat-shielding sheath 540.
In the above-described assembly, the plug boot 510 is covered with a heat-shielding sheath 520. Therefore, the load applied by the plug boot 510 and the heat-shielding sheath 520 is directly applied to the fitting zone of spark plug P. When the internal-combustion engine vibrates in such a state, both the high-voltage terminal press-fitted to the end portion of ignition cable K within the plug boot 510 and the stud terminal Pt of spark plug P which is connected to the high-voltage terminal suffer from wear. Consequently, the plug boot 510 can become detached from spark plug P, and the internal-combustion engine may malfunction.